1. Field of the Invention
The field of representative embodiments of this disclosure relates to methods and systems for adaptive noise cancellation (ANC), and in particular to an ANC feedback controller in which the feedback response is provided by a fixed transfer function feedback filter and a variable response filter.
2. Background of the Invention
Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as MP3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events.
In many noise cancellation systems, it is desirable to include both feed-forward noise cancellation by using a feed-forward adaptive filter for generating a feed-forward anti-noise signal from a reference microphone signal configured to measure ambient sounds and feedback noise cancellation by using a fixed-response feedback filter for generating a feedback noise cancellation signal to be combined with the feed-forward anti-noise signal. In other noise cancellation systems, only feedback noise cancellation is provided. An adaptive feedback noise cancelling system includes an adaptive filter that generates an anti-noise signal from an output of a sensor that senses the noise to be canceled and that is provided to an output transducer for reproduction to cancel the noise.
In any ANC system having a feedback noise-canceling path, the secondary path, which is the electro-acoustic path at least extending from the output transducer that reproduces the anti-noise signal generated by the ANC system to the output signal provided by the input sensor that measures the ambient noise to be canceled, determines a portion of the necessary feedback response to provide proper noise-canceling. In ANC systems in which the acoustic environment around the output transducer and input sensor varies greatly, such as in a mobile telephone where the telephone's position with respect to the user's ear changes the coupling between the telephone's speaker and a microphone used to measure the ambient noise, the secondary path response varies as well. Since the feedback path transfer function for generating a proper anti-noise signal is dependent on the secondary path response, it is difficult to provide an ANC controller that is stable for all possible configurations of the acoustic path between the output transducer and input sensor that may be present in an actual implementation.
Therefore, it would be desirable to provide an ANC controller with improved stability in ANC feedback and feed-forward/feedback ANC systems.